High temperature resistant filtration materials have been prepared by needling fine denier Nomex or fiberglass into supportive scrim made from the same material. When Nomex was utilized it was necessary to calender or otherwise compact the material after needling in order to obtain the desired air permeability. During usage said material tended to swell back to its precalendered density, resulting in leakage of fine dust particles and lowered efficiency.
In the case of filtration material made entirely of fibrous glass, it is not necessary to calender such material and high efficiencies were obtained. However, as a result of the poor flex strength of glass fiber, the longevity of this material is limited.